In efforts to avoid some of the drawbacks associated with polysilicon gate electrodes, replacement damascene metal gate processes have been devised. A damascene metal gate process forms a device with a disposable (dummy) gate, with a source, drain, spacer, etc., as in conventional processing. The disposable gate and dielectrics are etched away, exposing an original gate oxide. The disposable polysilicon gate is then replaced by a metal gate to achieve the lower resistivity provided by the metal material. General reference with respect to a replacement gate process can be made, as one non-limiting example, to commonly assigned U.S. Pat. No. 7,091,118 B1, “Replacement Metal Gate Transistor with Metal-Rich Silicon Layer and Method of Forming Same”, James Pan, John Pellerin, Linda R. Black, Michael Chudzik and Rajarao Jammy.
In silicon on insulator (SOI) technology a thin silicon layer is formed over an insulating layer, such as silicon oxide, which in turn is formed over a bulk substrate. This insulating layer is often referred to as a buried oxide (BOX) layer or simply as a BOX. Sources and drains of field effect transistors (FETs) are formed by the addition of N-type and/or P-type dopant material into the thin silicon layer, with a channel region being disposed between the source and drain.
It has become desirable to achieve a low threshold voltage (Vt) for transistors that operate with a scaled (reduced) value of Vdd.